It is well-known that mice infected by the parasite Toxoplasma gondii lose their innate aversion to cats. New research is now starting to shed more light on the precise nature of the behavioral changes that this tiny unicellular organism elicits in rodents as it literally takes control of their brain.

Fans of zombies and walking dead be warned: horror stories of mind-control exerted on unwitting beings by infectious pathogens are not confined to science-fiction novels and TV series. No need for post-apocalyptic settings or scientific experiments gone awry to make this happen: Mother Nature makes her own brew – and it is not for the fainthearted.

This is the case, in particular, of parasites that have several natural hosts through which they need to transit in order to reproduce themselves. Take the nematode Myrmeconema neotropicum (a kind of worm). It parasitizes Cephalotes atratus, a South American ant, modifying the way the ant looks in order to make it more likely to be eaten by birds, the next host in this parasite’s life cycle. How does it do this?

First, it transforms the infected abdomen of the ant (whose body is normally totally black) into a bright-red, berry-like ball. And that’s not all: it then “forces the ant to stand on leaves in a way that makes the altered abdomen visible”, says Cristina Afonso, a researcher in the Neurobiology of Action Lab at the Champalimaud Centre for the Unknown. Inevitably, the infected ants get mistaken for juicy berries and are eaten by birds, thus efficiently transporting the parasite to this host.

Afonso herself is interested in another kind of parasite, Toxoplasma gondii, and together with her colleagues has recently published a paper in the journal Scientific Reports which sheds light on how this common parasite induces behavioral changes in rodent hosts in order to facilitate its own transmission.

But more on this later. Afonso talks about other eery mind-controlling bugs, such as Dricocoelium dendriticum, which also infects ants, but whose ultimate host is cattle (cows, sheep and the like). How does D. dendriticum alter the infected ants’ behavior in order to expose them to the ruminants’ appetite? It makes them climb to the top of grass blades and stand still up there until the inevitable happens… Talk about zombie-ants.

“There are also macro-parasites, like the jewel wasp, which needs to lay its egg inside a cockroach in order for it to have food as it develops”, she further explains. These wasps, which have a brilliant emerald coloration, will sting a cockroach directly in the brain, delivering a cocktail of poison that will make it unable to initiate voluntary movement. Then, similarly to a dog on a leash, the wasp will grab the cockroach by its antennae and guide it to a burrow. The jewel wasp will then lay its egg inside the cockroach’s body, cover the burrow – and leave the cockroach to die, consumed by the new wasp as it develops.

Even more extreme is the case of caterpillars which, after acting as meal for still another parasitic wasp’s eggs, remain alive long enough to defend the wasp pupae from predators until the adult insect emerges!

Fearless mice

Back to Toxoplasma gondii. This is a unicellular parasite that can infect almost any warm-blooded animal, including humans: pregnant women are routinely tested for toxoplasmosis. In mice, infection by T. gondii is known to reduce aversive behavior with respect to cats, probably facilitating their capture by felines, where this parasite reproduces sexually (in other hosts like the mouse, it replicates by clonal division). However, the behavioral modifications that result from T. gondii infection are complex and may serve to increase overall capture probability of infected animals by many predators, not just cats.

Chronic infection of mice by T. gondii leads to the presence of parasite-containing cysts inside neurons. And, as Afonso and her colleagues showed in a previous study, published in 2012 in the journal PLoS ONE, when cysts are present in the mouse brain, these animals start moving faster and for longer periods of time.

“The way the mice locomote changes”, says Afonso. “Normally, mice scurry – they move a bit, then stop, then move, then stop. But infected animals move continuously, they have longer movement bouts. In the wild, this will attract more predator attention.”

Their reaction to exposed areas also changes. “They become risk-takers and are not afraid of open spaces, which goes against their nature as prey”, explains Afonso. Normally, mice are much more cautious.

In the 2012 study, using various experimental setups combined with statistical analysis, the authors compared the behavior of control (uninfected) mice with that of infected animals and discovered that the latter were not simply hyperactive, as could have been the case, but clearly presented a risk-fear behavior alteration. “The infected animals have a riskier behavior, they are actually less fearful, they don’t just move more”, says Afonso.

In one setup in particular, called the “elevated plus maze”, a cross-shaped box which stands at a height of a half-meter over the floor and consists of two covered and two uncovered, unprotected crossing arms, the team observed that infected mice were almost willing to jump off the open arms, whereas controls avoided these arms, confining themselves to the closed spaces.

Trapped!

But the scientists wanted to explore this further: were infected mice unable to perceive danger or, having perceived it, were they unable to stop acting recklessly? In their latest study they used, for the first time, a setup in which they could unexpectedly trap the mice inside a tunnel where the animals could barely move.

What they then observed was that, after the entrapment, infected mice were more willing to go back into the tunnel than control mice. And although the infected mice’s behavior suggested, according to the authors, that they had perceived the trapping as unpleasant, it seemed they didn’t mind facing this aversive event again. Infected animals just couldn’t adjust their behavioral response afterwards. “They didn’t care”, says Afonso. “Imagine these animals in the wild”, she adds. “They have difficulties activating a cautious behavior.” In other words, the risk-fear behavioral defects seem to affect the selection and execution of appropriate behavior and not the perception of a given event in the environment.

Dopamine involved?

Another issue the scientists wanted to probe further concerned the biochemical mechanisms through which Toxoplasma gondii manages to elicit these behavioral changes. This is still far from being understood, but there is a candidate substance: dopamine.

“Dopamine is one of the main neurotransmitters in the brain and over the years, it has been proposed as a strong candidate for explaining the effects of T. gondii, since infected animals have higher levels of dopamine in the brain”, says Afonso. “As a modulator of motor performance”, she adds, “dopamine would tie in really well” with the motor alterations observed in infected mice.

Even more so because, actually, T. gondii brain cysts secrete an enzyme, called tyrosine hydroxylase (TH), which transforms the amino acid tyrosine (one of the building blocks of proteins) into L-Dopa, which is, in turn, a precursor of dopamine.

To test the involvement of parasite-derived tyrosine hydroxylase, the authors compared the behavior, not only of infected versus uninfected mice, but also of mice infected with parasites that had a functional TH gene (which commands production of TH) versus parasites where this gene had been knocked out. This allowed them to determine whether the observed behavioral modifications were dependent on the presence of this gene in the parasite. They found that these two groups of infected mice were indistinguishable from one another in terms of their altered behavior.

In other words, it does not seem likely that the parasite’s ability to interfere with dopamine levels is the one mechanism through which T. gondii achieves behavioral modifications in infected mice. “We can’t say that a dopamine-dependent mechanism is the culprit”, concludes Afonso.

“Dopamine could be involved mainly through mechanisms other than parasite secretion”, according to her. It could be part of a more systemic physiological mechanism of action of T. gondii, involving not only the central nervous system, but also, for instance, the immune system. “There is a huge immune response to the infection”, says Afonso, “which could translate into brain effects. It’s not as simple as we thought.”

An estimated 40% of the world’s population is infected with T. gondii, which is mainly transmitted through the ingestion of contaminated food. Could the parasite also elicit behavioral changes in people?

“In humans, this infection has been correlated with certain conditions such as schizophrenia, suicide, car accidents (risky decisions taken by drivers or pedestrians), increased sexual activity in women, and the prevalence of deviant sexual behavior”, replies Afonso. “But no causal link has been established.”

Ana Gerschenfeld works as a Science Writer at the Science Communication Office at the Champalimaud Neuroscience Programme